In water treatment machinery, the core differences between primary RO and secondary RO lie in desalination accuracy, system configuration, application scenarios, and operating parameters. Here is a detailed comparison:
1、 Differences in desalination principles and processes
1. First level reverse osmosis
Principle: After pre-treatment (sand filtration, carbon filtration, softening, etc.), the raw water is subjected to a first stage reverse osmosis membrane to achieve desalination under high pressure.
Membrane components intercept dissolved salts, bacteria, organic matter, etc. in water, producing first grade fresh water, and directly discharging or partially recovering concentrated water.
Typical process:
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Raw water → pretreatment → primary reverse osmosis → fresh water (reduced salt content)
2. Secondary reverse osmosis
Principle: The first stage reverse osmosis produced water is used as the inlet water, and then the second stage reverse osmosis membrane is used for secondary desalination to further reduce the ion content in the water.
Secondary RO usually requires the addition of an intermediate water tank and a secondary high-pressure pump after the first stage of water production to increase the inlet pressure.
Typical process:
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Raw water → Pre treatment → Primary reverse osmosis → Intermediate water tank → Secondary reverse osmosis → High purity fresh water
2、 Comparison of core parameters
Indicator Level 1 Reverse Osmosis Level 2 Reverse Osmosis
The desalination rate of single-stage membrane is about 95%~98% (depending on the quality of the raw water), and the desalination rate can reach over 99% after the superposition of two-stage membranes
Conductivity of produced water quality: 5-50 μ S/cm (suitable for ordinary desalination needs) Conductivity: ≤ 1 μ S/cm (close to pure water level)
The inlet water requires pre-treatment to SDI ≤ 5, residual chlorine ≤ 0.1mg/L, and water temperature of 15-35 ℃. The first stage RO production water is used as the inlet water, and the water quality has been greatly optimized
The working pressure is 0.8~1.5 MPa (depending on the salt content of the raw water), and the pressure of the secondary membrane is slightly lower than that of the primary membrane (about 0.6~1.2 MPa)
The single-stage recovery rate of water utilization is about 60%~80% (high proportion of concentrated water discharge), and the total recovery rate of the two-stage system can reach 75%~90% (concentrated water cascade utilization)
Number of membrane elements: Single set of membrane elements (e.g. 6 pieces/system) Two sets of membrane elements (primary+secondary, e.g. 6+4 pieces/system)
3、 System configuration differences
1. First level reverse osmosis
Key components:
Preprocessing equipment (sand filter, carbon filter, precision filter)
Single stage high-pressure pump (1 unit)
Single set reverse osmosis membrane module
Concentrated water discharge valve (without intermediate water tank)
characteristic:
The system is simple, cost-effective, and suitable for scenarios with moderate water quality or low desalination requirements.
Concentrated water has a high salt content and requires attention to environmental discharge (such as for irrigation or further treatment).
2. Secondary reverse osmosis
Key components:
Preprocessing+Level 1 RO (same level system)
Intermediate water tank (storing primary produced water and buffering water quality fluctuations)
Secondary high-pressure pump (to increase pressure to meet the requirements of the secondary membrane)
Secondary reverse osmosis membrane components (usually using membranes with higher desalination rates, such as low-energy membranes)
Concentrated water reflux design (secondary concentrated water can be refluxed to the primary inlet water to improve the overall recovery rate)
characteristic:
The system is complex and costly (equipment investment increases by 30%~50%), but the purity of the produced water is significantly improved.
The secondary membrane has a lower load and a longer lifespan (which can be extended by 1-2 years compared to the primary membrane).
4、 Comparison of application scenarios
1. Applicable scenarios for first level reverse osmosis
Industrial production:
Boiler makeup water (hardness ≤ 0.03mmol/L)
Circulating cooling water (reducing the risk of scaling)
Food processing water (such as beverage blending water)
Civilian field:
Direct drinking water in residential areas (conductivity ≤ 100 μ S/cm)
Primary treatment of seawater desalination (combined with other processes)
2. Applicable scenarios for secondary reverse osmosis
High purity water demand:
Electronics industry: Chip cleaning water (resistivity ≥ 18M Ω· cm, EDI required)
Pharmaceutical industry: Injection water (compliant with USP and Chinese Pharmacopoeia standards)
Chemical industry: lithium battery electrolyte preparation, high-purity reagent production
Special water quality treatment:
Reuse of high salt wastewater (if the TDS of concentrated water is still>1000mg/L after first stage RO, further desalination by second stage RO is required)
Laboratory ultra pure water system (as a pretreatment stage)
5、 Maintenance and Cost Comparison
Project Level 1 Reverse Osmosis Level 2 Reverse Osmosis
The membrane cleaning frequency is high (impurities in the raw water directly contact the membrane) and low (the inlet water of the secondary membrane has been purified)
Consumables replacement cost, pre-treatment filter material/membrane replacement cost, moderate increase in secondary membrane and intermediate water tank maintenance cost
Running a single-stage high-pressure pump with energy consumption of approximately 1-3 kWh/m ³, a two-stage pump with an energy consumption increase of 30% -50%
Small footprint (without intermediate water tank) and large footprint (requiring additional space to install water tank and secondary pump)
Summary: How to choose?
Choose a first level RO: If the goal is to reduce water hardness, remove most of the salt, and the water production standard is not higher than “conductivity<50 μ S/cm”, such as ordinary industrial water or drinking water.
Choose secondary RO: If extreme desalination is required (such as conductivity<1 μ S/cm), or if the salt content of the raw water is extremely high (such as TDS>3000mg/L), high-purity water production needs to be achieved through two-stage membranes.
Note: In practical applications, secondary RO is often combined with EDI (Electrodeionization) or polishing mixed bed to further remove trace ions and achieve ultra pure water standards (such as water used in the semiconductor industry).
Post time: May-16-2025